Add support for suspending handler methods in WebFlux

This commit turns Coroutines suspending methods to `Mono` which can be
handled natively by WebFlux.

See gh-19975

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,8 @@
 
 package org.springframework.web.reactive.result.method;
 
+import static org.springframework.web.reactive.result.method.InvocableHandlerMethodKt.*;
+
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.lang.reflect.ParameterizedType;
@@ -27,6 +29,7 @@ import java.util.stream.Stream;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.DefaultParameterNameDiscoverer;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ParameterNameDiscoverer;
 import org.springframework.core.ReactiveAdapter;
@@ -48,6 +51,7 @@ import org.springframework.web.server.ServerWebExchange;
  *
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
+ * @author Sebastien Deleuze
  * @since 5.0
  */
 public class InvocableHandlerMethod extends HandlerMethod {
@@ -136,7 +140,13 @@ public class InvocableHandlerMethod extends HandlerMethod {
 			Object value;
 			try {
 				ReflectionUtils.makeAccessible(getBridgedMethod());
-				value = getBridgedMethod().invoke(getBean(), args);
+				Method method = getBridgedMethod();
+				if (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(method.getDeclaringClass())) {
+					value = invokeHandlerMethod(method, getBean(), args);
+				}
+				else {
+					value = method.invoke(getBean(), args);
+				}
 			}
 			catch (IllegalArgumentException ex) {
 				assertTargetBean(getBridgedMethod(), getBean(), args);

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ContinuationHandlerMethodArgumentResolver.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2002-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.reactive.result.method.annotation;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.web.reactive.BindingContext;
+import org.springframework.web.reactive.result.method.HandlerMethodArgumentResolver;
+import org.springframework.web.server.ServerWebExchange;
+
+/**
+ * No-op resolver for method arguments of type {@link kotlin.coroutines.Continuation}.
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+public class ContinuationHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver {
+
+	@Override
+	public boolean supportsParameter(MethodParameter parameter) {
+		return "kotlin.coroutines.Continuation".equals(parameter.getParameterType().getName());
+	}
+
+	@Override
+	public Mono<Object> resolveArgument(MethodParameter parameter, BindingContext bindingContext, ServerWebExchange exchange) {
+		return Mono.empty();
+	}
+}

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ControllerMethodResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,7 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodIntrospector;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.annotation.AnnotatedElementUtils;
@@ -190,6 +191,9 @@ class ControllerMethodResolver {
 		result.add(new RequestAttributeMethodArgumentResolver(beanFactory, reactiveRegistry));
 
 		// Type-based...
+		if (KotlinDetector.isKotlinPresent()) {
+			result.add(new ContinuationHandlerMethodArgumentResolver());
+		}
 		if (!readers.isEmpty()) {
 			result.add(new HttpEntityArgumentResolver(readers, reactiveRegistry));
 		}

spring-webflux/src/main/kotlin/org/springframework/web/reactive/result/method/InvocableHandlerMethod.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2002-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.reactive.result.method
+
+import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.reactor.mono
+import reactor.core.publisher.onErrorMap
+import java.lang.reflect.InvocationTargetException
+import java.lang.reflect.Method
+import kotlin.reflect.full.callSuspend
+import kotlin.reflect.jvm.kotlinFunction
+
+/**
+ * Invoke an handler method converting suspending method to {@link Mono} if necessary.
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+internal fun invokeHandlerMethod(method: Method, bean: Any, vararg args: Any?): Any? {
+	val function = method.kotlinFunction!!
+	return if (function.isSuspend) {
+		GlobalScope.mono { function.callSuspend(bean, *args.sliceArray(0..(args.size-2)))
+				.let { if (it == Unit) null else it} }
+				.onErrorMap(InvocationTargetException::class) { it.targetException }
+	}
+	else {
+		function.call(bean, *args)
+	}
+}